Aqueous latexes of a variety of homopolymers and copolymers have been known in the art for many years. Among the polymers which are useful in producing aqueous latexes one can mention homo- and copolymers of acrylate monomers such as alkyl acrylates and alkyl methacrylates; vinyl esters such as vinyl acetate; vinyl or vinylidene halides such as vinyl chloride and vinylidene chloride; acrylonitrile; isoprene; chloroprene; butadiene; isobutylene; and styrene. These monomers are mentioned as being merely illustrative and not all-inclusive of monomers useful in the production of aqueous latexes.
The aqueous latexes are produced by the well known techniques of emulsion polymerization. Generally, emulsion polymerization involves the addition of the monomer or omomers to be polymerized to a stirred aqueous reaction medium which contains a surfactant or emulsifying agent. Additionally, a polymerization catalyst or initiator is employed which may be partially or completely added to the medium prior to the introduction of the polymerizable monomer or monomers.
The aqueous polymer latexes are stable and valuable articles of commerce, being widely employed, for example, in the field of paints, coatings, adhesives, and sealants. The aqueous polymer latexes of acrylic monomers, particularly homopolymers of lower alkyl acrylates or copolymers of lower alkyl acrylates with other comonomers such as styrene, alkyl methacrylates, higher alkyl acrylates, acrylic or methacrylic acid, vinyl halides and vinylidene halides, and the like, have become especially widely used in recent years in the production of water-based paints.
In recent years, considerable effort has been expended toward obtaining so-called "high solids" latexes, that is, latexes having a high concentration ratio of polymer to water. Generally, it is more meaningful to discuss the solids content of latexes in terms of volume percent solids than weight percent solids. The theoretical maximum amount of polymer one can have in a latex and still have a fluid is dictated by volumetric limitations, that is, it is a matter of efficiency of polymer particle packing. Since different polymers vary in density, latexes of different polymers may have considerably different weight percent solids contents even though they have the same efficiency of polymer particle packing, that is, the same volume percent solids content. Thus, the maximum volume percent solids content of a latex is essentially the same for all polymers, whereas a given weight precent solids content which constitutes a relatively high solids content for one particular polymer may constitute a relatively low solids content for a different polymer of higher density. In the past twenty years the solids content of commercially produced latexes has been increased from about 40 to about 60 volume percent. However, the industry still desires to achieve higher solids contents, on the order of 70 volume percent or more, in aqueous latexes.
High solids contents are considered desirable because they reduce shipping and storage costs, allow increased productivity of plant equipment, reduce drying time for applied latexes, and allow the application of films of any desired thickness in fewer passes. Moreover, coating and paint formulators who add pigments to the aqueous latexes prefer to have as much of the water which will be in the final formulation available for use in the pigment dispersion as is possible.
Thos skilled in the art know that particles of varying size can be packed more efficiently on a volumetric basis than particles of a single size. Latex containing polymer particles having a variety of particle sizes are referred to as "polydisperse" latexes. However, a process which produces a polydisperse latex does not necessarily guarantee high solids content. For example, a polydisperse latex can be prepared by separately producing two or more "monodisperse" latexes of different polymer particle size and blending the latexes thus produced. While polydispersity is achieved, the advantages of more efficient particle packing are not realized because no reduction in water content accompanies latex blending.
Ideally, a process for producing a high solids latex should achieve two objectives:
it should provide for the production of polymer particles of broad size distribution which distribution is carefully controllable by the operator; and it must obtain the desired distribution in a manner which practices strict water economy, that is, the process must employ a minimum of water so that the packing efficiency which is possible with particles of broad size distribution is fully realized.
While recent improvements in latex production processes have increased somewhat the solids contents obtainable in aqueous latexes, none of the processes completely achieves both the foregoing objectives, as a result of which aqueous latexes have not been consistently produced above about 60-65 volume percent solids.
U.S. Pat. No. 3,637,563 discloses a method for increasing the solids content of an aqueous latex by forming a pre-emulsion comprising a continuous phase of monomer having a discontinuous phase of about 6-25 wt.% water therein and continuously feeding the pre-emulsion to an equeous reaction medium containing a polymerization catalyst. This process achieves certain economies in the consumption of water, but the polymer particle size distribution is essentially limited to the naturally occurring distribution.
Great Britain patent No. 1,191,649 claims a method of producing a high solids latex which entails carrying out the emulsion polymerization to about 50-90% completion in the presence of a bare minimum of catalyst and surfactant and then adding surfactant and catalyst before completing polymerization. This process results essentially in a "bimodal" particle size distribution, which provides for somewhat more efficient particle packing than a monodisperse latex, however, it does not provide a broad controlled particle size distribution which optimizes particle packing efficiency.
U.S. Pat. No. 3,424,706 discloses a process for producing "polydisperse" latexes of polyvinylidene chloride in which several incremental additions of a pre-formed seed latex are made to the reaction mixture during the polymerization reaction. Essentially, the effect of this process is to produce a latex having particles of several discreet size ranges, each particle size corresponding to one of the incremental additions of monomer. It is said, at Col. 6, lines 3-4 therein that the addition of seed latex can be continuous. This patent provides a significantly broader distribution of polymer particle sizes than the preceding art, however, it does so at the expense of water economy. In this patent, a monodisperse seed latex is added to the reaction mixture during polymerization. Generally, seed latexes are about 10 to 20 volume percent solids. In Example 1 of U.S. Pat. No. 3,424,706, the seed latex is 20 weight percent solids (about 15 volume percent, based on the density of polyvinylidene chloride). This means that for every volume unit of new seed latex particles added to the reactor during the reaction, 5 to 6 additional volume units of water must be added to the reactor.